Material heating method

ABSTRACT

A material heating method includes: a step of charging a silicon-containing carbon steel material into a heating furnace; a step of preheating the material; a first heating step of raising the temperature of the material; a second heating step of lowering the temperature of the material so as to reduce a temperature difference between the surface and inside of the material; a third heating step of raising the temperature of the material; and a soaking step of reducing the temperature difference between the surface and inside of the material. The temperature of the material in the heating furnace is maintained at the melting point of fayalite or lower.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of PCT/KR2011/010012 filed on Dec. 22, 2011, which claims priority to Korean Application No. 10-2011-0051139 filed on May 30, 2011, which applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a material heating method, and more particularly, to a material heating method which is capable of improving the surface quality of a material.

BACKGROUND ART

In the case of an external plate for vehicles and a plate for domestic products, a surface quality serves as an important control factor, and a scale defect is considered as a serious problem of hot-rolled steel. Recently, car makers and companies to deal with various electronic products have delicate requirements for the surface quality of hot-rolled steel.

In order to improve the surface quality, a descaler is installed to remove scale on the surface of hot-rolled steel during a hot rolling process. However, there exist scale defects which are not removed by the descaler.

The above-described configuration is a related art for helping an understanding of the present invention, and does not mean a related art which is widely known in the technical field to which the present invention pertains.

SUMMARY

Embodiments of the present invention are directed to a material heating method capable of improving the surface quality of a material. Embodiments of the present invention are directed to a material heating method capable of removing red scale.

Embodiments of the present invention are directed to a material heating method capable of improving the surface quality of a material containing carbon of 0.15 to 1.2 wt % and silicon of 0.10 wt % or more.

In an embodiment, a material heating method includes: a step of charging a silicon-containing carbon steel material into a heating furnace; a step of preheating the material; a first heating step of raising the temperature of the material; a second heating step of lowering the temperature of the material so as to reduce a temperature difference between the surface and inside of the material; a third heating step of raising the temperature of the material; and a soaking step of reducing the temperature difference between the surface and inside of the material. The temperature of the material in the heating furnace is maintained at the melting point of fayalite or lower.

The material may contain carbon of 0.15 to 1.2 wt % and silicon of 0.1 wt % or more.

A temperature T₁ of a first heating zone of the heating furnace for performing the first heating step, a temperature T₂ of a second heating zone of the heating furnace for performing the second heating step, a temperature T₃ of a third heating zone of the heating furnace for performing the third heating step, and a temperature T₄ of a soaking zone of the heating furnace for performing the soaking step may be set to satisfy the relation of T₂<T₄<T₃<T₁.

The temperature T₁ may range from 1,190 to 1,210° C. The temperature T₂ may range from 1,130 to 1,160° C. The temperature T₃ may range from 1,170 to 1,190° C.

The temperature T may range from 1,160 to 1,180° C.

The temperature T1 may be set to 1,200° C., the temperature T₂ may be set to 1,150° C., the temperature T₃ may be set to 1,180° C., and the temperature T₄ may be set to 1,170° C.

A residence time of the material may range from 160 to 230 minutes.

A temperature difference ΔT₁ between the surface and inside of the material after the first heating step, a temperature difference ΔT₂ between the surface and inside of the material after the second heating step, a temperature difference ΔT₃ between the surface and inside of the material after the third heating step, and a temperature difference ΔT₄ between the surface and the inside of the material after the soaking step may satisfy the relation of ΔT₄<ΔT₂ and ΔT₃<ΔT₁.

In accordance with the embodiment of the invention, the material heating method may significantly improve the surface quality of the material through control of the heating process. In particular, the material heating method may remove red scale of a material containing carbon of 0.15 to 1.2 wt % and silicon of 0.1 wt % or more.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features and advantages of the invention will become apparent from the following detailed description in conjunction with the accompanying drawings, in which:

FIG. 1 is diagram illustrating the temperature profile of a material and the temperature of a heating furnace in accordance with an embodiment of the present invention;

FIG. 2 is a cross-sectional view of the heating furnace in accordance with the embodiment of the present invention;

FIG. 3 is a perspective view of a rolling apparatus in accordance with the embodiment of the present invention; and

FIG. 4 is a cross-sectional view of an interfacial structure of the material in accordance with the embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the invention will hereinafter be described in detail with reference to the accompanying drawings. It should be noted that the drawings are not to precise scale and may be exaggerated in thickness of lines or sizes of components for descriptive convenience and clarity only. Furthermore, the terms as used herein are defined by taking functions of the invention into account and can be changed according to the custom or intention of users or operators. Therefore, definition of the terms should be made according to the overall disclosures set forth herein.

FIG. 1 is diagram illustrating the temperature profile of a material and the temperature of a heating furnace in accordance with an embodiment of the present invention. FIG. 2 is a cross-sectional view of the heating furnace in accordance with the embodiment of the present invention. FIG. 3 is a perspective view of a rolling apparatus in accordance with the embodiment of the present invention. FIG. 4 is a cross-sectional view of an interfacial structure of the material in accordance with the embodiment of the present invention.

The present invention relates to a method for heating a silicon-containing carbon steel material, and more particularly, to a method for heating a material containing carbon (C) of 0.15 to 1.2 wt % and silicon (Si) of 0.10 wt % or more.

In the present invention, an object to be heated in a heating furnace and an object subjected to a hot rolling process are collectively referred to as ‘material’. That is, the material may refer to slab, bloom, billet and the like, which are manufactured through a continuous casting process. Furthermore, the slab or bloom is supplied to the heating furnace and then heated to perform a roughing rolling process, a finishing rolling process or the like. The material may also be used as a term indicating a bar after the roughing rolling process or a strip after the finishing rolling process.

Silicon is a ferrite stabilizing element and serves to improve the activity of carbon. During a heat treatment, silicon activates movement of carbon within cementite in perlite, and reduces the content of carbon within the cementite, thereby improving toughness and ductility. Furthermore, silicon is added as a deoxidizer for removing oxygen within steel during a steel manufacturing process, and employed in the ferrite so as to increase the strength.

However, when silicon is added into steel, red scale may occur during a hot rolling process. The red scale is not descaled through a descaler, thereby reducing the surface quality of hot-rolled steel. In particular, high carbon steel containing carbon of 0.15 to 1.20 wt % has a higher silicon content than general steel, in order to improve heat treatment efficiency. That is, high carbon steel may have a high silicon content of 0.10 wt % or more, or particularly, 0.30 wt %. In this case, a large amount of red scale may occur during the hot rolling process.

It is known that the red scale occurs due to fayalite (Fe₂SiO₄) formed on the surface of a material. In accordance with the embodiment of the present invention, while a material passes through a heating furnace, the temperature of the material is maintained at the melting point of fayalite, that is, about 1,173° C. or less, in order to suppress the occurrence of fayalite. Furthermore, the heating condition in the heating furnace may be controlled to reduce a temperature difference between the surface and inside of the material, thereby preventing the occurrence of scale defects including the red scale. Thus, a subsequent rolling process may be smoothly performed even through the heating temperature of the material is lowered.

Referring to FIGS. 1 and 2, the material heating method in accordance with the embodiment of the present invention includes a preheating step (not illustrated), a first heating step, a second heating step, a third heating step, and a soaking step. In a material temperature-time graph of FIG. 1, a solid line indicates temperature changes of the inside (central portion) of the material, and a dashed dotted line indicates temperature changes of the surface of the material.

The preheating step corresponds to a preheating zone F₀ of the heating furnace 100, the first heating step S₁ corresponds to a first heating zone F₁ of the heating furnace 100, the second heating step S₂ corresponds to a second heating zone F₂ of the heating furnace 100, the third heating step S₃ corresponds to a third heating zone F₃ of the heating furnace 100, and the soaking step S₄ corresponds to a soaking zone F₄ of the heating furnace 100.

The preheating step (not illustrated) is to preheat the material S charged into the furnace 100. The preheating step may correspond to a section for preheating the material using heat (hot wind) coming out of heating devices 112, 116, 120, and 124 of the heating furnace, which are installed for the heating steps S₁, S₂, and S₃ and the soaking step S₄ performed after the preheating step. That is, the preheating zone F₀ of the heating furnace 100 may include no heating devices provided at the top and bottom of the material S. However, depending on the structure of the heating furnace 100, the preheating zone F₀ may include a heating device.

The first heating step S₁ is to raise the temperature of the material having passed through the preheating step, that is, to heat the material at a predetermined temperature so as to roll the material. At the first heating step S₁, the temperature of the heating furnace 100 is set to T₁. Since the material S is heated from the surface, the temperature of the surface of the material S is maintained to be higher than the temperature of the inside of the material S, and the heat is transmitted from the surface to the inside of the material S such that the temperature of the inside of the material S gradually rises. After the first heating step S₁, a difference in temperature between the surface and inside of the material S corresponds to ΔT₁.

In the present embodiment, the temperature of the inside of the material S indicates the temperature of a point positioned at the center of thickness, the center of length, and the center of width of the material S, or specifically, the center of gravity of the material S.

The second heating step S₂ is to reduce the temperature difference between the surface and inside of the material S by lowering the temperature of the heating furnace. For this operation, while the second heating step S₂ is performed, the temperature of the heating furnace 100 is set to T₂ lower than in the first heating step S₁ (T₂<T₁). Since the material S is cooled down from the surface, the temperature difference between the surface and inside of the material decreases after a predetermined time. That is, after the second heating step S₂, the temperature difference ΔT₂ between the surface and inside of the material S becomes smaller than ΔT₁.

The third heating step S₃ is to raise the temperature of the material S. In order to prevent the occurrence of red scale, the temperature of the material S, that is, the temperatures of the surface and inside of the material may be maintained at the melting point T_(m) of fayalite or lower. However, the temperature of the material may be maintained at as high temperature as possible, in order to reduce a rolling load. Thus, during the third heating step

S₃, while the temperature of the material S is maintained at the melting point T_(m) of fayalite or lower, the temperatures of the surface and inside of the material may be raised to as high temperature as possible, in order to smoothly perform a rolling process. For this operation, the temperature T₃ of the heating furnace may be set to be higher than T₂ and lower than T₁.

After the third heating step S₃, a temperature difference ΔT₃ between the surface and inside of the material S may be smaller or larger than ΔT₂, but may be maintained to be smaller than ΔT₁. That is, the temperature difference ΔT₃ between the surface and inside of the material S may differ depending on the heating time (process time) and the temperature of the furnace in the second heating step S₂, the heating time and the temperature of the furnace in the third heating step S₃, and the shape and property of the material S.

The soaking step S₄ is to finally lower the temperature difference between the surface and inside of the material S. Even at the soaking step S₄, the temperatures of the surface and inside of the material S may be maintained at the melting point T_(m) of fayalite or lower, and the temperature difference between the surface and inside of the material may be reduced as much as possible. Then, it is possible to suppress the occurrence of scale including red scale, reduce the rolling load, and prevent rolling defects caused by the temperature difference between the surface and inside of the material. That is, after the soaking step S₄, the temperature difference ΔT₄ between the surface and inside of the material may be controlled to be minimized. Specifically, the relation between the temperature differences may be set at ΔT₄<ΔT₂ and ΔT₃<ΔT₁. Ideally, ΔT₄ may be set to 0° C. However, a slight temperature difference may occur due to the limit of the manufacturing process.

The total process time required for the preheating step, the first heating step S₁, the second heating step S₂, the third heating step S₃, and the soaking step S₄, that is, the residence time may be set in the range of 160 to 230 minutes. When the residence time is less than 160 minutes, it is difficult to reduce the temperature difference between the surface and inside of the material while the material S is heated at the melting point T_(m) of fayalite or lower. When the residence time exceeds than 230 minutes, it is not desirable in terms of productivity. Thus, the residence time may be set in the range of 160 to 230 minutes, or desirably, 180 to 200 minutes.

Specifically, within the range of the residence time, the preheating step may be set to 50±10 minutes, the first heating step S₁ may be set to 30±10 minutes, the second heating step S₂ may be set to 25±10 minutes, the third heating step S₃ may be set to 35±10 minutes, and the soaking step S₄ may be set to 40±10 minutes. However, the residence times of the respective heating steps and the soaking step are not limited thereto.

While the material S passes through the heating furnace 100, the temperature of the material S must be maintained at the melting point T_(m) of fayalite or lower. For this operation, the temperature T₁ of the first heating zone, the temperature T₂ of the second heating zone, the temperature T₃ of the third heating zone, and the temperature T₄ of the soaking zone may be set to satisfy the relation of T₂<T₄<T₃<T₁. Then, it is possible to suppress the occurrence of scale including red scale, reduce the temperature difference between the surface and inside of the material, and improve the rolling efficiency.

The temperature relation is only an example. As long as the temperature of the material S is maintained at the melting point T_(m) of fayalite or lower, the method for setting the temperature of the heating furnace is not limited thereto.

Specifically, the temperature of the first heating zone F₁ in which the first heating step S₁ is performed to heat the material S may be set in the range of 1,190 to 1,210° C., or desirably, set to 1,200° C. In the above-described temperature range, the material S may be heated for 30±10 minutes. The temperature of the second heating zone F₂ in which the second heating step S₂ is performed may be set in the range of 1,130 to 1,160° C., or desirably, set in the range of 1,140 to 1,150° C., or more desirably, set to 1,150° C. In the above-described temperature range, the material S may be heated for 25±10 minutes. The temperature of the third heating zone F₃ in which the third heating step S₃ is performed may be set in the range of 1,170 to 1,190° C., or desirably, set to 1,180° C. In the above-described temperature range, the material S may be heated for 35±10 minutes. The temperature of the soaking zone F₄ in which the soaking step S₄ is performed may be set in the range of 1,160 to 1,180° C., or desirably, set to 1,170° C. In the above-described temperature range, the material S may be heated for 40±10 minutes.

Referring to FIG. 2, the heating furnace 100 in accordance with the embodiment of the present invention includes the preheating zone F₀, the first heating zone F₁, the second heating zone F₂, the third heating zone F₃, and the soaking zone F₄.

The material S is charged through a charging part 102, and extracted from the heating furnace through an extracting part 104. The charging part 102 may be opened/closed through a charging door 106, and the extracting part 104 may be opened/closed through an extracting door 108.

The heating furnace 100 may additionally include a skid beam 110 successively arranged from the charging part 102 and the extracting part 104, and the material S to be heated is received on the skid beam 110 and then transferred. The skid beam 110 may include a fixed beam for supporting the material S and a working beam for advancing the material S. While the material S is heated inside the heating furnace 100, the working beam may lift, advance, lower, or retreat the material S, thereby gradually transferring the material S toward the extracting part 104 from the charging part 102.

The first heating zone F₁ includes a first heating device 112 and a first temperature measuring device 114 to measure the temperature of the first heating zone F₁. The second heating zone F₂, the third heating zone F₃, and the soaking zone F₄ may include a second heating device 116 and a second temperature measuring device 118, a third heating device 120 and a third temperature measuring device 122, and a fourth heating device 124 and a fourth temperature measuring device 126, respectively. The preheating zone F₀ may include a heating device and a temperature measuring device or may not include a heating device and a temperature measure device. Alternatively, the preheating zone F₀ may not include a heating device, but may include a temperature measuring device. The positions of the temperature measuring devices 114, 118, 122, and 124 are not limited, but the temperature measuring devices 114, 118, 122, and 124 may be installed on a cover at the top of the heating furnace 100.

Each of the first heating device 112, the second heating device 116, the third heating device 120, and the fourth heating device 124 may include a burner which burns gas such as heavy oil, natural gas, or coke oven gas (COG) to raise the temperature of the heating furnace 100 using hot wind, or an IR heater. However, the burner has an advantage in terms of cost. In the present embodiment, a plurality of burners may be installed. Furthermore, the burner may be installed at the top and bottom sides of the material S or installed at any one of the top and bottom sides of the material S.

The first temperature measuring device 114, the second temperature measuring device 118, the third temperature measuring device 122, and the fourth temperature measuring device 126 may be not limited to specific devices, as long as they can measure temperature. However, the first temperature measuring device 114, the second temperature measuring device 118, the third temperature measuring device 122, and the fourth temperature measuring device 126 may include a thermocouple.

According to the initial temperature of the material S and the final temperature required during extraction, the target temperatures, the temperature-rising speeds, and the residence times of the preheating zone F₀, the first to third heating zones F₁ to F₂, and the soaking zone F₄ may be controlled.

When the temperature of the material S is maintained at the melting point of fayalite or lower and the temperature T₁ of the first heating zone, the temperature T₂ of the second heating zone, the temperature T₃ of the third heating zone, and the temperature T₄ of the soaking zone are set to satisfy the relation of T₂<T₄<T₃<T₁, it is possible to suppress the occurrence of scale including red scale, reduce the temperature difference between the surface and inside of the material, and improve the rolling efficiency.

Referring to FIG. 3, the rolling apparatus in accordance with the embodiment of the present invention may include a heating furnace 100, a slab sizing press 200, a roughing mill 210, an edge heater 220, a descaler 230, a finishing mill 240, a run-out table 250, a cooler 260, and a winder 270.

The heating furnace 100 is a reheating furnace which reheats the material S so as to hot-roll the material S. Since the structure and process condition of the heating furnace 100 have been described above, the detailed descriptions thereof are omitted herein.

The slab sizing press 200 is a width roller which reduces a width difference in the longitudinal direction of the material S and rolls the material S to a predetermined width according to an end user's request. The roughing mill 210 rolls the material S to a suitable thickness and width required during a finishing rolling process. The movement of the material S from an input side to an output side of the roughing mill 210 or from the output side to the input side of the roughing mill 210 refers to a pass. Such a pass may be performed a plurality of times, and a standby time may be set to reduce a temperature gradient of the material S after each pass.

The edge heater 220 may be installed to prevent a drop in temperature of the edge of the material S, and the descaler 230 may remove scale on the surface of the material S using high-temperature water.

The finishing mill 240 is a device which manufactures a steel plate in a final shape based on a thickness and width required by a customer or required during a hot rolling process. The material S having passed through the finishing mill 240 may be cooled down to a target temperature through a laminar flow of cooling water coming out of the cooler 260, while passing through the run-out table 250, and then wound by the winder 270. While the material S is cooled down, air may be used instead of the cooling water.

The above-described rolling apparatus is only an embodiment. A part of the devices constituting the rolling apparatus may be omitted, and other devices may be additionally included. For example, a descaler may be added into the rolling apparatus before and after the roughing mill. For another example, the descaler may be added before and after the slab sizing press 200. For another example, an edger mill may be added to uniformize a width difference caused by the slab sizing press. Furthermore, the names of the above-described devices are attached for convenience of description, and the devices may have different names.

Referring to FIG. 4, the steel plate (material) may have scale on the surface of base steel A, the scale being composed of wustite (FeO) B, magnetite (Fe₃O₄) C, and hematite (Fe₂O₃) D which are sequentially formed. The material heating method in accordance with the embodiment of the present invention may lower the temperature of the material to the melting point of fayalite or lower and thus suppress the occurrence of fayalite or reduce the adhesion between the base steel A and the wustite B even though fayalite occurs between the base steel A and the wustite B.

The fayalite is generated through a reaction of silicon oxide (SiO₂) from silicon existing in the base steel A and the wustite B. Since the fayalite has high adhesion to the base steel A, the fayalite is not removed through the descaler, thereby causing scale after the rolling process. In particular, since the adhesion between the fayalite and the base steel A significantly increases while the fayalite is molten and cooled, the fayalite may be not removed through the descaler.

In accordance with the embodiment of the present invention, as the temperature of the material in the heating furnace is maintained at the melting point of fayalite or lower, the occurrence of fayalite may be suppressed or the adhesion of the fayalite may be significantly reduced. Thus, scale may be easily removed through the descaler, and the surface quality of the material after the rolling process may be significantly improved.

Furthermore, although the scale is not removed or regenerated after the rolling process, the scale may be easily removed during a pickling process or the like.

Although some embodiments have been provided to illustrate the invention in conjunction with the drawings, it will be apparent to those skilled in the art that the embodiments are given by way of illustration only, and that various modifications and equivalent embodiments can be made without departing from the spirit and scope of the invention. The scope of the invention should be limited only by the accompanying claims. 

1. A material heating method comprising: a step of providing a silicon-containing carbon steel material in a heating furnace; a step of preheating the material; a first heating step of raising the temperature of the material; a second heating step of lowering the temperature of the material so as to reduce a temperature difference between the surface and inside of the material; a third heating step of raising the temperature of the material; and a soaking step of reducing the temperature difference between the surface and inside of the material, wherein the temperature of the material in the heating furnace is maintained at the melting point of fayalite or lower.
 2. The material heating method of claim 1, wherein the material contains carbon of 0.15 to 1.20 wt % and silicon of 0.10 wt % or more.
 3. The material heating method of claim 1, wherein a temperature T₁ of a first heating zone of the heating furnace for performing the first heating step, a temperature T₂ of a second heating zone of the heating furnace for performing the second heating step, a temperature T₃ of a third heating zone of the heating furnace for performing the third heating step, and a temperature T₄ of a soaking zone of the heating furnace for performing the soaking step are set to satisfy the relation of T₂<T₄<T₃<T₁.
 4. The material heating method of claim 3, wherein the temperature T₁ ranges from 1,190 to 1,210° C.
 5. The material heating method of claim 3, wherein the temperature T₂ ranges from 1,130 to 1,160° C.
 6. The material heating method of claim 3, wherein the temperature T₃ ranges from 1,170 to 1,190° C.
 7. The material heating method of claim 3, wherein the temperature T ranges from 1,160 to 1,180° C.
 8. The material heating method of claim 3, wherein the temperature T1 is 1,200° C., the temperature T₂ is 1,150° C., the temperature T₃ is 1,180° C., and the temperature T₄ is 1,170° C.
 9. The material heating method of claim 1, wherein a residence time of the material ranges from 160 to 230 minutes.
 10. The material heating method of claim 1, wherein a temperature difference ΔT₁ between the surface and inside of the material after the first heating step, a temperature difference ΔT₂ between the surface and inside of the material after the second heating step, a temperature difference ΔT₁ between the surface and inside of the material after the third heating step, and a temperature difference ΔT₄ between the surface and the inside of the material after the soaking step satisfy the relation of ΔT₄<ΔT₂and ΔT₃<ΔT₁. 